1. Field of the Invention
The present invention relates generally to an apparatus and method for generating light energy, and more particularly, to passively modelocked lasers having the emission wavelength controlled by balancing non-linear effects.
2. State of the Art
Practically speaking, ultra-short pulse sources which can consistently emit pulses having pulse durations less than one picosecond should be compact, environmentally stable and require relatively low power. A document entitled "Additive-Pulse-Compression Modelocking of a Neodymium Fiber Laser", by Fermann M. E. et al, Optics Letters, Vol. 16, No. 4, Feb. 15, 1991, pages 244-246, describes a passively modelocked fiber laser for generating ultra-short pulses using a rare-earth doped fiber.
An environmentally stable, passively modelocked laser is described in co-pending U.S. application Ser. No. 08/169,707 entitled "Environmentally Stable Passively Modelocked Fiber Laser Pulse Source", filed in the U.S. Patent and Trademark Office on Dec. 20, 1993, by Dr. Martin E. Fermann and Dr. Donald J. Hatter, the disclosure of which is hereby incorporated by reference in its entirety. As referenced therein, the phrase "environmentally stable" refers to a pulse source which is substantially immune to a loss of pulse generation due to environmental influences such as temperature drifts and which is, at most, only slightly sensitive to pressure variations.
Exemplary embodiments of an environmentally stable, ultra-short pulse source have been implemented by differentially exciting two linearly polarized, fundamental eigenmodes of a highly birefringent fiber (HBF) such that they accumulate a differential non-linear phase delay after a particular propagation distance. Due to interference of the eigenmodes at a polarizer, the non-linear phase delay translates into an amplitude modulation, which can provide sufficient pulse-shortening per round-trip to produce stable passive modelocking. The amount of amplitude modulation is sensitive to the linear phase delay between the two interfering eigenmodes.
Linear phase drifts between two polarization eigenmodes of a cavity, such as the cavity described in accordance with exemplary embodiments of the previously mentioned co-pending application, can be eliminated using a pigtailed Faraday rotator mirror (FRM) as one of the cavity mirrors. A document entitled "Single-Polarisation Fibre Amplifier", by I. N. Duling III et al, Electronics Letters, Jun. 4, 1992, Vol. 28, No. 12, pages 1126-1128 also generally describes using a Faraday rotator mirror a as an end mirror. The Faraday rotator mirror permits environmental stability to the amplifier.
Passively modelocked lasers are typically subject to a variety of processes that affect the output pulses. For example, in a document entitled "Mode Locking In Solitary Lasers", by T. Brabec et al, Optics Letters, Vol. 16, No. 24, Dec. 15, 1991, pages 1961-1963, pulse formation in modelocked lasers is described wherein the presence of isolated (i.e., discrete) cavity elements results in the occurrence of instability. Noticeable affects on stability can also result from third-order dispersion, and such instabilities can result in the formation of spectral sidebands as described, for example, in a document entitled "Characteristic Sideband Instability of the Periodically Amplified Average Soliton", by S. M. J. Kelly, Electronics Letters, Vol. 28, page 806, 1992. Further, in a document entitled "Ultrabroad-Band Femtosecond Lasers", by Christian Spielmann et al, Journal of Quantum Electronics, instabilities due to third-order dispersion which give rise to asymmetric pulse spectra are described.
Thus, while conventional passively modelocked lasers are typically subject to processes which can result in instabilities that affect the output pulses (e.g., emission wavelength), any gain-pulling due to these instabilities is minimal such that the spectrum of the modelocked pulses remains located close to the peak of the gain profile. In particular, the tuning range of standard modelocked lasers is governed dominantly by their finite gain bandwidth and cannot be extended or controlled by non-linear processes.
Because the tuning range of standard modelocked lasers is primarily governed by their finite gain bandwidth, any ability to control selection of a particular emission wavelength of the modelocked laser is substantially limited. Accordingly, it would be desirable to provide a tunable laser having an emission wavelength which can be controlled over a relatively broad bandwidth.